This invention relates to covers carried by the tips of turbine buckets and brush seals surrounding and engaging the bucket covers, and more particularly relates to seals between the adjoining edges of bucket covers for minimizing or eliminating radial outflow of the fluid medium flowing through the turbine onto the bristles of the surrounding brush seal.
As set forth in U.S. Pat. No. 6,036,437, of common assignee herewith, turbine bucket covers and brush seals surrounding the bucket covers for sealing between upstream and downstream portions of a turbine stage are known. Turbine bucket covers are typically provided in either of two forms. A bucket and a discrete bucket cover are often integrally cast one with the other and applied to the turbine wheel. Adjoining edges of the adjacent covers typically form a generally Z-shaped joint as viewed in a radial direction. Other bucket covers are provided in the form of a cover band which spans two or more buckets. The bucket cover bands typically have openings for receiving tenons formed on the tips of the buckets and which tenons are peened to secure the cover bands to the bucket. Like the integrally cast buckets and covers, the adjoining edges of the cover bands engage one another, generally in a Z-shaped joint when viewed radially.
As will be appreciated, the pressure of the fluid medium within the flow path through the turbine, for example, steam flowing in the steam path of a steam turbine, is at a significantly higher pressure than the steam pressure radially outwardly of the covers. This difference in pressure causes radial outflow of the fluid medium through the gaps between the edges of adjoining covers. These gaps permit both radial and axial outflow of the fluid medium and turbine performance is thereby degraded. Labyrinth seals and/or brush seals are typically employed in the shroud surrounding the buckets and covers and those seals can limit the performance losses attributable to axial flow through and around the gaps. Brush seals also afford an added performance benefit over the use of labyrinth seals. However, turbine bucket tip brush seals are affected by radial flow in the region of the bristle pack. The radial flow into the bristle pack through the radial gaps between the bucket covers causes bristle fatigue and failure. That is, the discrete flows of the fluid medium radially outwardly through the gaps between adjacent bucket covers tend to deflect the bristles in an outward direction, as well as to splay the bristles in an axial direction. Thus, for every pass of a gap by a given bristle location, the bristles are deflected both axially and radially by the radial flow. Moreover, the bristles attempt to recover before the next radial outward flow repeats those deflections. These deflection cycles cause bristle fatigue and failure. It will be appreciated that for a 60 Hz machine having eighty covers and gaps in a stage, a 40,000 Hz stimulus is provided to the bristle pack at the various circumferential locations of the bristles about the turbine wheel.
In accordance with a preferred embodiment of the invention, the gaps between the adjoining edges of the covers are sealed to the extent that the radial flow leakage is minimized or eliminated. These portions of the edges of the adjoining covers interlock with one another such that radial leakage, e.g., steam, from the flow path is minimized or prevented, with the result that turbine performance is enhanced. By sealingly engaging the cover portions to one another along the area of contact between the tips of the bristles and the outer cover surfaces, the potentially damaging effect on the seal bristle pack by the radial outflow is eliminated. Particularly, the adjoining edges of the covers are overlapped with one another. For example, lap joints are formed between the adjoining edges. In another form, tongue-and-groove joints may be provided. In a still further form, additional cover segments may overlie the covers in staggered circumferential relation to the underlying covers such that the gaps between adjoining underlying covers are sealed by the overlying covers.
In another preferred embodiment, in a turbine having a plurality of rotatable buckets, a stationary shroud surrounding the rotatable buckets, and a fluid medium flow path through the turbine, sealing between the buckets and the stationary shroud is accomplished by a plurality of arcuate covers carried by radial outer ends of the buckets for rotation with the buckets about an axis of the turbine in a predetermined circumferential direction, the covers having leading and trailing edges in the direction of rotation of the buckets, a brush seal comprising a plurality of bristles projecting from the shroud for sealingly engaging outer surfaces of the covers, and a joint between circumferentially adjacent covers sealing adjoining leading and trailing edges thereof to one another, thus substantially precluding radial outflow of the fluid medium onto the bristles as the buckets and covers rotate relative to the brush seal.
In a further preferred embodiment, in a turbine having a plurality of rotatable buckets, a stationary shroud surrounding the rotatable buckets, and a fluid medium flow path through the turbine, sealing between the buckets and the stationary shroud is accomplished by a plurality of arcuate covers carried by radial outer ends of the buckets for rotation with the buckets about an axis of the turbine in a predetermined circumferential direction, the covers having adjoining edges, a brush seal comprising a plurality of bristles projecting from the shroud for sealingly engaging outer surfaces of the covers, and a joint between circumferentially adjacent covers including at least portions of the adjoining edges overlapping one another and sealingly engaging the adjacent covers to one another at axial locations corresponding to axial locations of the cover surfaces engaged by the bristles, to substantially preclude radial outflow of the fluid medium past the overlapping portions onto the bristles.